Cant say I've heard of them needing only to be sexed after a year. if someone has the correct probes and is very very careful it can be done, but not for the weak or heart.
One word of advice i can give you is you get what you pay for. If the Ammie your getting is really cheap in price, there is an excellent chance they are wild caught and parasite ridden. W/c snake are usually the ones that are very nasty. C/b seem to be "more handleable" I have 3 that are w/c and would tear your face off if you let them. I have one c/b girl and she's actually pretty nice(for an ammie).
The best pics I have seen are on
www.colrallus.com they have some fabulous pics and some great info on them.
They are awesome snakes, but definately not a lap snake.